Although mutator factors, including such elements as the male recombination (MR) factor in Drosophila melanogaster, occur in natural populations, we still have little information about their frequencies or the effects they have upon the genetic structure of populations. Our study has two primary sets of objectives. First, we are surveying Drosophila populations to determine the frequency of mutator factors, concentrating at first upon strains showing MR activity. MR activity has been found in all of the more than 20 populations we have sampled, and was found in almost all of the individuals in each population. Second, we are attempting to understand the mechanism by which the MR mutator factor produces its effects. Male recombination in the MR strains apparently involves some premeiotic trigger event and can occur in clusters. Chromosome breakage in germ cells has been proposed as the central event to explain the sterility, mutator activity, and other expressions of MR activity. Evidence is accumulating that MR factors are microorganisms. Finally, MR mutator activity is expressed only upon outcrossing a MR strain with a different population. The consequences of this observation has for our understanding of mutator gene effects and mutation frequencies in natural populations is being studied in great detail.